This invention relates to control apparatus for controlling the operation of a compressor in a compressor-driven system to avoid stall or surge in the system and to improve efficiency. The invention is susceptible of widely diverse use and may be applied to any system having equipment driven by a compressor, such as a turbine, pump, fan, jackhammer, electric generator, jet engine, etc. It is particularly useful when incorporated in refrigeration equipment and will be described in that environment.
Various systems have been developed for preventing surge in large capacity turbocompressors, such as of the centrifugal or axial type. Surge or surging is an undesirable condition that may occur when, for example, the gas velocity through the compressor drops below a given minimum. It is an oscillation or the onset of oscillation in which the gas flow will speed up and slow down alternately, and it may even reverse its direction through the compressor. Surging causes excessive vibrations and noise, and may cause permanent damage to the compressor. Unfortunately, all of the previous attempts to avoid surge have substantially lessened the compressor efficiency. For example, in one prior control system, when the possibility of surging exists gas is bypassed from the discharge side to the suction side or is merely passed through a blow-off line. In another system, guide vanes at the compressor inlet (usually called prerotation vanes or PRV) are adjusted, when a surging condition is approached, to introduce drag or resistance to the gas flow in an amount sufficient to prevent surging.
These prior solutions to the surge problem employ passive and inefficient mechanical mechanisms that dissipate considerable energy. The surge prevention arrangement of the present invention, on the other hand, constitutes a significant improvement over the systems developed heretofore since essentially no energy is needed to keep the compressor-driven system out of surge. Moreover, additional economies are realized in that the construction of the compressor may be simplified considerably from that which is usually required.